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Does a parasite have a better chance of survival if an Inuit or a Mayan spits on it?
1Department of Oral Biology, Faculty of Dentistry, McGill University, Montreal, Quebec, Canada. shields@medcor.mcgill.ca
Abstract:
An earlier puzzling observation [Shields and Mann, J Craiofac Genet Dev Biol, 16:126-136, 1996] that the prevalence of a polymorphic male predominate, major salivary gland-associated, static lesion of the mandible increased exponentially from the Arctic to the Tropics was explained by both positive and negative selection (balancing) on major salivary gland endocrine and exocrine factors. Additional prevalence rates presented here identified three high prevalence high-Temperate zone cultures that were unusually exposed to alimentary parasites. A correlation between macroparasite exposure and the mandibular lesion helped refine the potential selective forces that fashioned major salivary gland size variation. The data suggests that positive selection occurred for androgen-induced enlargement of the suite of major salivary glands and consequently increased quantities of factors. Increased quantities of salivary gland biologically active factors enhance innate protection against infestation of macroparasites per se, especially gut parasites. The data further suggests that negative selection against enlarged salivary glands occurred as protection against electrolyte imbalances in electrolyte stressed environments and in females.
Insights
Salivary gland size variation, linked to a mandibular lesion, is explained by balancing selection. Increased gland size offers parasite protection but poses risks in electrolyte-stressed environments and for females.
Area of Science:
- Evolutionary biology
- Human genetics
- Parasitology
Background:
- A previous study noted an exponential increase in a specific mandibular lesion's prevalence from Arctic to Tropical regions.
- This lesion is associated with major salivary glands and exhibits male predominance.
Observation:
- New prevalence data reveal three high-prevalence populations in temperate zones with high exposure to alimentary parasites.
- A correlation was found between macroparasite exposure and the mandibular lesion.
Findings:
- The study refines the understanding of selective forces influencing major salivary gland size.
- Positive selection favors androgen-induced salivary gland enlargement, increasing protective factors against parasites.
- Negative selection acts against enlarged glands in electrolyte-stressed environments and in females.
Implications:
- Salivary gland size variation is a complex adaptation influenced by parasite defense and environmental constraints.
- Findings contribute to understanding human adaptation and disease prevalence in diverse geographical locations.